Schedule I - Condensed Financial Information Of Parent (Statement of Income) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Condensed Financial Statements, Captions [Line Items]
Operating revenues	$ 1,370	$ 1,670	$ 1,419	$ 1,594	$ 1,322	$ 1,638	$ 1,403	$ 1,475	$ 6,053		$ 5,838		$ 5,781
Operating expenses									4,799		4,654		4,593
Operating Income	125	561	322	246	171	567	261	185	1,254		1,184		1,188
Interest income from affiliates									10	[1],[2]	3	[1]	4	[1],[3]
Other Nonoperating Income (Expense)									57		43		33
Interest charges									341		398		392
Income taxes (benefit)									377		311		307
Net income attributable to Ameren Corporation - continuing operations	46	294	150	97	48	305	105	54	587		512		516
Net Income (Loss) Attributable to Ameren Corporation - Discontinued Operations	2	(1)	(1)	(1)	(11)	(3)	(10)	(199)	(1)		(223)		(1,490)
Net income (loss) attributable to Ameren Corporation	48	293	149	96	37	302	95	(145)	586		289		(974)
Comprehensive Income from Continuing Operations
Pension and other postretirement activity, net of income taxes (benefit)									(12)		30		(8)
Comprehensive Income (Loss) from Continuing Operations, Net of Tax, Portion Attributable to Parent									575		542		508
Other Comprehensive Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest											(18)		58
Comprehensive Income (Loss) from Discontinued Operations									(1)		(242)		(1,440)
Comprehensive Income (Loss) Attributable to Ameren Corporation									574		300		(932)
Other Comprehensive Income (Loss), Taxes:
Pension and other postretirement activity, tax (benefit)									(7)		16		(6)
Parent Company
Condensed Financial Statements, Captions [Line Items]
Operating expenses									11		26		17
Operating Income									(11)		(26)		(17)
Equity in earnings of subsidiaries									607		546		546
Interest income from affiliates									3		3		3
Other Nonoperating Income (Expense)									2		(5)		(4)
Interest charges									16		42		39
Income taxes (benefit)									(2)		(36)		(27)
Net income attributable to Ameren Corporation - continuing operations									587		512		516
Net Income (Loss) Attributable to Ameren Corporation - Discontinued Operations									(1)		(223)		(1,490)
Net income (loss) attributable to Ameren Corporation									586		289		(974)
Comprehensive Income from Continuing Operations
Pension and other postretirement activity, net of income taxes (benefit)									(12)		30		(8)
Comprehensive Income (Loss) from Continuing Operations, Net of Tax, Portion Attributable to Parent									575		542		508
Other Comprehensive Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest									0		(19)		50
Comprehensive Income (Loss) from Discontinued Operations									(1)		(242)		(1,440)
Comprehensive Income (Loss) Attributable to Ameren Corporation									574		300		(932)
Other Comprehensive Income (Loss), Taxes:
Pension and other postretirement activity, tax (benefit)									$ (7)		$ 16		$ (6)

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

[2] Includes Ameren Illinois' interest income received in 2014 relating to the 2013 and 2014 IEIMA revenue requirement reconciliation regulatory assets.

[3] Includes Ameren Missouri's interest income relating to a refund of charges included in an expired power purchase agreement with Entergy. See Note 2 – Rate and Regulatory Matters for additional information.